This application is based on and claims the benefit of German Patent Application No. 10146807.5 filed Sep. 22, 2001, which is incorporated by reference herein.
The invention relates to a method for producing longitudinally welded helically corrugated metal tubing.
Examined German Application DE-AS 1 086 314 discloses a method representative of the field of the invention. With this prior art method, only tubing with a relatively flat corrugation can be produced. The reason for this is that the width of the corrugation valley cannot be smaller than the thickness of the disk-shaped corrugation tool. If the pitch is large, i.e., if the distance between the corrugation valleys is large, the corrugation depth is approximately equal to the depth by which the corrugation tool dips into the smooth tubing. If the pitch is made very small, the corrugation peak is pulled into the tubing as the corrugation tool dips into the smooth tubing. As a result, there is a dimensional relationship between the maximum corrugation depth and the pitch. Even selecting an extremely thin corrugation tool does not necessarily result in a tight pitch, since there is a lower limit of the width of the corrugation valley due to the toughness of the tubing material. For this reason, the prior art method is limited with respect to the flexibility of the resulting tubing.
To increase the flexibility of corrugated metal tubing, it has been attempted to stress the previously corrugated metal tubing after production, e.g., as described in published German application DE-PS 493 930.
From German publication DE 24 00 842 C it is known to corrugate the tubing under pressure in the direction of the longitudinal axis regardless of the forces applied for corrugation. For this purpose, the tubing is slowed after corrugation. The apparatus used for this purpose is provided with an annular tool of a variable diameter directly behind the corrugation unit to apply frictional forces and thereby compressive stresses in the direction of the longitudinal axis. The drawback of this method is that it is impossible to apply uniformly high frictional forces during the entire production process. As a consequence, the corrugation as seen over the length of the tubing is uneven.
An object of the present invention is to provide a method that makes it possible to produce tubing with a deeper corrugation or a smaller pitch, while the corrugation is uniform over the length of the tubing.
This object is attained partially by a feed device that engages the corrugated tubing directly behind the corrugation unit, where the feed rate of the feed device is slower than the advancing rate of the corrugated tubing.
The method according to the invention is particularly advantageous for the sheathing of plastic optical fibers that are used for signal transmission in motor vehicles.